SCOPE 21 -The Major Biogeochemical Cycles and Their Interactions 


C, N, P, and S Cycles: Major Reservoirs and Fluxes

2.1 Introduction
2.2 The Carbon Cycle
2.3 The Nitrogen Cycle
2.3.1 General
2.3.2 Distribution of Nitrogen
2.3.3 Fluxes of Nitrogen
2.4 The Phosphorus Cycle
2.5 The Sulphur Cycle


The present chapter brings together the most recent information on reservoir sizes and fluxes for the four major cycles with which we are concerned. Internal consistency between different cycles has been aimed for to facilitate comparisons. Variability between different estimates as quoted represents uncertainty in our knowledge about the relevant reservoirs and processes.

The tables and diagrams given are not the result of re-assessment of the different cycles, but should rather be considered as reference material that might facilitate the reading of the remainder of the book. References should therefore be made to the original data sources instead of the present chapter.

This overview was not available in its present form at the time of the workshop and some of the authors of the following chapters do not explicitly refer to the present compilation but rather to the original research on which it was based. It is judged that this synthesis still will be of value.



The global carbon cycle has been described in detail in SCOPE Report 13 (Bolin et al, 1979), where references to earlier attempts to present a consistent picture of this cycle are also given. Some new data have become available during the last few years, but they hardly warrant a more critical re-analysis of earlier views. There are, however, uncertainties and inconsistancies in that budgets do not always balance and further work is required to resolve some of these problems. It should also be emphasized that we need to develop methods of how to describe the carbon cycle in more detail, i.e. the way the transfer of carbon within the ocean is accomplished or the cycling of carbon within the various main biomes, including the soil. This is dealt with in several of the following chapters. The objective of this note is to give a simple view of the overall transfers that govern the global carbon cycle. We shall thus merely give a synthesis of the data given in SCOPE 13, with some minor modifications to serve as background for the discussions of biogeochemical interactions.

Table 2.1 gives the distribution of carbon between the major reservoirs. For the atmosphere a few different values are given, referring to the likely pre-industrial content of the atmosphere, the accurately known values for 1959, when careful measurements began and the most recent data from 1980.

Carbon reservoirs of the oceans are also reasonably well known, while there are considerable uncertainties about the size of the carbon reservoirs in the terrestrial biosphere, the pedosphere and the lithosphere. In some cases different estimates are quoted and a best estimate finally chosen. Data for the lithosphere are not accurate to more than one significant figure, even though sometimes two are given in the table. The values depend on the use of proper values for the average amount of carbon in different rocks, estimates of their relative occurrence and the thickness of the crust.

Table 2.2 summarizes in a similar manner the magnitude of the fluxes between the different reservoirs. Again a best estimate is given based on the determinations quoted.

Figures 2.1 and 2.2 display in schematic form the carbon cycle. The exchanges between the atmosphere, the terrestrial biomass, soil, rivers and the oceans including marine life as shown in Figure 2.1 are relatively large compared with the reservoirs involved. Characteristic turn-over times vary from a few weeks for marine biota to about one thousand years for the ocean. The circulation in the rock cycle involves reservoirs that are one to ten million times larger. Since the fluxes are one or two orders of magnitude less than those at work in air and ocean transport and in photosynthesis, the characteristic turn-over times are hundreds of millions of years.

Table 2.1 Major carbon reservoirs (when not otherwise indicated, in units of 1015 g C = PgC). References are given after Table 2.2



Carbon dioxide
Pre-industrial 265 ppm 560 (1)
290 ppm 615 (1)
1959 316 ppm 670 (2)
1980 336 ppm 712 (2)
Methane 1.41 ppm 3 (3)
Carbon monoxide 0.11 0 .2 (3)
Other C-containing gases 0
.05 (3) 
total (1980) 715


Dissolved inorganic carbon 37,400 (4)
Dissolved organic carbon 1,000 (5)
Particulate organic carbon 30 (5)
Biota 3
Ocean, total 38,500

Terrestrial biota and pedosphere

Phytomass short-lived 130 830 (7)
long-lived 700 560 (1)
Animals 1 (3)
Man 0.03  (8)
Bacteria 2 (8)
Fungi 1 (8)
Standing dead organic carbon 30 (8)
Litter 60 (8)
Peat (very uncertain) 160 (1),(8)
Soil organic carbon (excluding peat) 1,500 (8),(9)
Organic carbon in
biota and pedo-
sphere, total 2,3002,600


Continental crust
Sediments, carbonate 26 x 106 (10)
Sediments, non-carbonate* 10 x 106 (10)
Igeneous rock, non-carbonate 79 x 106 (10)
Igeneous rock, non-carbonate 1.1 x 106 (10)
Oceanic crust
Sediments, carbonate 14 x 106 (10)
Sediments, non-carbonate* 6.0 x 106 (10)
Basalt, carbonate 0.3 x 106 (10)
Basalt, non-carbonate 0.3 x 106
Lithosphere, total 65 x 106

*These reservoirs contain the oil, gas and coal reserves of which  510 x 103 units (510 x 1018 Pg) can possibly be recovered for use.

 Table 2.2 Major carbon fluxes (when not otherwise indicated in units of 1015 g C yr-1 = Pg yr-1)

Atmosphere, terrestrial biosphere Reference

Carbon monoxide production 1.2 ± 0.6 (3)
Methane production 0.6+0.3 (3)
Photosynthesis 53 (7)
60 (8)
Litterfall 50 ± 10 (8)
Grass and forest fires (natural; gross) 24 (4, 11)
Animal (herbitore) consumption 6 (8)
Decay of organic matter = photosynthesis
Fluxes induced by man due to:
Fossil fuel combustion 1980 5.2 (12)
Fossil fuel combustion 18601980 165 (12)
Deforestation 1970 (net) 1.0 ± 0.5       2.5 (13)
Agriculture 1970 (net) 1.5 ± 1.0
Deforestation, agriculture 18601970 150 (13)


Air-sea exchange, average rate 60 ± 15 mmole/m2 (ppm)yr (4)
1860 CO2 concentration 290 ppm 75 ± 20 (4)
1980 CO2 concentration 336 ppm 90 ± 20 (4)
Net flux atmosphere to sea, 1980 2.5 (4) 

Oceans, marine biota

Primary production 40 ± 10 (6)
Detritus fall out from surface
layer(~ 75 m) 4 (1),(6)
Catch of fish 0.006 (6)

Freshwater, oceans, lithosphere

River flux, inorganic, dissolved 0.5 (14)
River flux, inorganic, particulate 0.2 (14)
River flux, organic, dissolved 0.30.7 (15)
River flux, organic, particulate 0.20.5 (15)
Sedimentation in continental basins 0.05 (13)
Weathering on land 0.3 (10)
Glacial erosion 0.03 (10)
Marine Erosion 0.005 (10) 
Metamorphosis (sediments
igneous rock) 0.008 (10) 
Subduction (marine sediments
igneous rock) 0.3 (10)
Sedimentation in sea, inorganic 0.15 (10)
Sedimentation in sea, organic 0.04 (10)


Bolin et al. (1979)


Ajtay et al. (1979)


Bacastow and Keeling (1981)


Schlesinger (1984)
(3)   Freyer (1979) (10)  Kempe (1979a,b)
(4)   Bolin et al. (1981) (11)  Seiler and Crutzen (1980)
(5)   Mopper and Degens (1979) (12)  Rotty (1981)
(6)   De Vooys (1979) (13)  Moore et al. (1981)
(7)   Whittaker and Likens (1975) (14)  Kempe (1979a) 
(15)  Meybeck (1981)


Figure 2.1 Size of reservoirs (in 1015 g) and fluxes (in 1015 g yr-1) for the part of the carbon cycle that is in a state of comparatively rapid turn-over, i.e. characteristic turn-over times less than about 1000 years

Figure 2.2 Fluxes (in 1015 g yr-1) for the carbon cycle in the earth's crust, where the characteristic turn-over times are of the order of 100 million years (based on Kempe, 1979a, b)



2.3.1 General

During the past decade there has been a rapid development towards a qualitative understanding of the global nitrogen cycle. We are, however, still far from being able to present a quantitative picture of the global nitrogen cycle. This inability is due to two main reasons. The first one is inherent in all attempts to construct models of the annual transport of elements between different reservoirs. In most cases the reported annual flows do not represent truly integrated values but are based on extrapolations to a global, yearly basis of determinations of flow rates at specific points in time and space. Secondly, there are still lacunes in our knowledge of the qualitative aspects of the biogeochemical nitrogen cycle, and certain previously neglected processes seem to be of major importance on a global scale. Examples of this are the recent rediscovery of the importance of nitrification in the production of nitrous oxide (Bremner and Blackmer, 1978; Cohen and Gordon, 1979; Crutzen, Chapter 3, this volume) and the possible production of nitric oxide also during nitrification (Lipschulz et al., 1981).

Figure 2.3 is a schematic representation of the biogeochemical nitrogen cycle. Unlike that of, for example, phosphorus, the atmosphere plays an important role in the biogeochemical nitrogen cycle on account of the importance of gaseous compounds (N2, N2O, NO, NH3) all of which can be produced and consumed through biotic and abiotic processes. A more accurate understanding of the biogeochemical nitrogen cycle has become of interest in recent years as a result of the importance of processes leading to the production of nitrogen oxides in view of the importance of such gases in the regulation of the chemical composition of the atmosphere.

2.3.2 Distribution of Nitrogen

The global distribution of nitrogen is shown in Table 2.3. Although nitrogen is present in relatively large concentrations in rocks, sediments, and the atmosphere, its availability in compounds that can be utilized by most forms of life is severely restricted. This deficiency in biologically available nitrogen in terrestrial and aquatic systems makes nitrogen one of the most important limiting nutrients.

In the atmosphere a minute fraction of nitrogen occurs in forms other than N2. The quantitatively most important form of combined nitrogen in the atmosphere is nitrous oxide (N2O), which accounts for 99.5% of all combined nitrogen. The small amounts of nitrogen compounds occuring in the atmosphere, do however, play a major role in regulating major processes in the atmosphere (see Crutzen, Chapter 3, this volume).

Figure 2.3 A global nitrogen cycle. Units are in Tg (1012g) N yr-1. From Söderlund & Rosswall (1982) based on Söderlund & Svensson (1976)

In the oceans, dimolecular nitrogen (in dissolved form) is also the most abundant form of nitrogen. Nitrate and nitrogen in dead organic matter occur in approximately equal amounts. Biomass nitrogen accounts for less than 0.001% of the total amounts of nitrogen in the hydrosphere. The ratio of plant: animal: microbial biomass-N (Table 2.3) is 15:8.5:1, which differs markedly from the ratios found in terrestrial biomass (25:0.4:1) in that animals (mainly zooplankton) contain an appreciable reservoir of nitrogen as compared to what is found in terrestrial systems.

The nitrogen in the lithosphere is inaccessible to living organisms except man, who, through the burning of coal, will release some of the bound nitrogen into the atmosphere.

Nitrogen in terrestrial systems occurs mainly in soil organic matter, litter and soil inorganic nitrogen (97% of total) with biomass accounting for only less than 3%. In the biomass, 95% occurs in the plants.

Table 2.3 Major nitrogen reservoirs (in units of 1015 g N = Pg N)

Atmosphere % of total Reference

Dimolecular nitrogen
3 900 000
> 99.999
Nitrous oxide
< 0.0001 (2)
< 0.0001 (2)
< 0.0001 (2)
Nitric oxide + Nitrogen dioxide (NOx)
< 0.0001 (2)
< 0.0001
Organic nitrogen 0.001 < 0.0001


Plant biomass
Animal biomass
Microbial biomass
0.00006 (5)
Dead organic matter (dissolved)
Dead organic matter (particulate)
0.010.1 (3)
Dimolecular nitrogen (dissolved)
22 000
Nitrous oxide
Ammonium 7 0.03

Pedosphere including biota

Plant biomass
Animal biomass
Microbial biomass
Soil: organic matter
 inorganic  160 34


190 000 000
       400 000
0.2 (8)
Coal deposits        120 0.00006

(1) Robinson and Robbins (1970)
(2) Galbally and Roy (in manuscript)
(3) Söderlund and Svensson (1976)
(4) Delwiche (1970)
(5) This compilation (Based on Mopper and Degens (1979) and a C/N ratio in micro-organisms of 12.5.)
(6) Emery et al. (1955)
(7) Delwiche and Likens (1977)
(8) Stevenson (1965)
(9) Donald (1960)

Simpson et al. (1977) made a compilation of data on nitrogen reservoir sizes from four different publications. Although the data in that compilation agree well with those in Table 2.3, there are estimates which differ by an order of magnitude. Galbally and Roy (in manuscript) and Sweeney et al. (1977), for example, estimated the atmospheric content of ammonia to be 1.7 and 1.5 Tg, respectively, while Delwiche and Likens (1977) gave a value of 28 Tg. The similarity between many of the published data is, however, dependent on the use of the same original source by many authors. In several instances the originally cited sources do not present the basis for the calculations and the correctness cannot be assessed. Unfortunately, there are few well documented reports on the distribution of nitrogen in different reservoirs.

2.3.3 Fluxes of Nitrogen

There is considerably uncertainty with regard to most estimates of process rates in the global biogeochemical nitrogen cycle (Table 2.4). Most estimates made in the past 10 years differ by an order of magnitude between lowest and highest values. In addition, the uncertainty is further increased because previously unidentified processes in the biogeochemical nitrogen cycle probably exist. Examples of such new processes are the mesospheric source of N2O from excited N2 as suggested by Zipf and Prasad (1982) and the possible production of NO during nitrification (Lipschulz et al., 1981). A further quantification of natural fluxes of nitrogen compounds between the oceanic-terrestrial systems and the atmosphere is needed in order for reliable atmospheric models involving nitrogen compounds to be developed. Here we only consider the exchanges of nitrogen compounds between the terrestrial, aquatic and atmospheric systems, while the internal transfers are not discussed. It should be realized that, for example, for the terrestrial systems, the internal transfers of nitrogen in the soilplant subsystem are one to two orders of magnitude larger than the transfers to and from the terrestrial systems (Rosswall, 1976).

Man is increasingly affecting the global biogeochemical nitrogen cycle. The industrial production of nitrogen fertilizer will, towards the end of this century, probably become as large as that produced through biological nitrogen fixation in the global terrestrial ecosystem (Söderlund and Svensson, 1976). Increased combustion temperatures increase the production of NOx, which contributes to the acidification of rain water. If we wish to determine the possible implications of such increased amounts of fixed nitrogen for the global cycles, it is essential that they are evaluated against background knowledge of the amounts of nitrogen that are parts of the natural biogeochemical nitrogen cycle. It should be evident from the data cited in Tables 2.3 and 2.4 that we are still far from having such a quantitative knowledge.

Table 2.4 Fluxes of nitrogen (Tg N yr-1) in the global biogeochemical nitrogen cycle. The ranges summarize rates given by the following authors: Delwiche (1970), Burns and Hardy (1975), Söderlund and Svensson (1976), McElroy et al. (1976), CAST (1976), Delwiche and Likens (1977), Liu et al. (1977), Hahn and Junge (1977), Sweeney et al. (1977), NAS (1978) and Bolin (1979). For a compilation of the individual estimates, see Rosswall (1981)

Tg N    

Biological nitrogen fixation: land 44200
Biological nitrogen fixation: ocean   1130
Atmospheric fixation (lightning) 0.530
Industrial fixation 60 


Industrial combustion and fossil fuel burning
     (NOx, N2O) 10-20


Fires 10200
Biogenic NOx production 090


Denitrification: land 43390
Denitrification: ocean 0330
N2O from denitrification: land 1669


N2O from denitrification: ocean 580 (5)
Nitrification N2O production: land ?       
Nitrification N2O production: ocean 410 (6)
Ammonia volatilization 36250
Dry and wet deposition NH3/NH4+ 110240
Dry and wet deposition NOx 40116
Dry and wet deposition organic-N 10100
River run-off (total N) 1340

(1) Data for 1979/80 (UN, 1981)
(2) Crutzen (Chapter 3, this volume)
(3) The lowest value refers to the estimated of NOx production from soils of 
     0.15 Tg given by Crutzen (Chapter 3, this volume).
(4) Söderlund and Svensson (1976).
(5) The lower estimate comes from Crutzen (Chapter 3, this volume) and   
      the upper from Söderlund and Svensson (1976).
(6) Cohen and Gordon (1979).


Previous evaluations of the global P cycle have identified the key fluxes and reservoirs, and have provided some estimates of their relative magnitudes (Stumm,1973; Lerman et al., 1975; Pierrou, 1976). A revision is provided here (Table 2.5; Figure 2.4). The use of phosphorus fertilizers in modern agriculture and the eutrophication of fresh waters from run-off and effluent discharge are the most visible results of human intervention in the P cycle.

The intent of this work is to summarize our current understanding of the dynamics of the phosphorus cycle by describing some of the chemical properties of P that affect its distribution, reviewing the assumptions used in the calculation of earlier P budgets and, where possible, up-dating them with new data (Table 2.5). It is becoming increasingly evident that element cycles cannot only be viewed in their global aggregate, but must be analysed on more relevant space scales. Therefore, the continental portion of the P cycle is analysed with finer resolution for 10 major geopolitical zones (Table 2.6).

Phosphate is liberated into the environment by the weathering of the apatite rocks. Because phosphate tends to precipitate to form materials of low solubility, sorb onto surfaces, and form complexes with metal ions, much of the phosphate released by weathering is immobilized (Van Wazer, 1973). As a result, of the total soil P, free phosphate is  often present in only trace quantities, with little leaching into fresh waters.

Phosphorus is present in the biota in a wide variety of organic compounds. These organic compounds are characterized by either fairly weak POC ester bonds or stable PC bonds, and undergo the hydrolytic degradation of esters and condensed polyphosphates. With these properties, P is critical as a mobile entity of cell metabolism and as a basic structural element of cell materials. Other than as an intermediate, phosphate does not participate in reductionoxidation reactions, as do C, N, and S. Because the ambient concentrations are low and the demands specific, phosphate becomes an important element for primary production in both terrestrial and aquatic environments.

The terrestrial biota contains much less P than do the source rocks and soils, with the largest reservoirs in the forests of North America, the U.S.S.R., Latin America, and Tropical Africa (450560 Tg P). Although the total dissolved reservoir in the oceans is about 77,000 Tg P, only about 50120 Tg P are contained in marine biota. This is due in large part to much of the P being below the euphotic zone.

The primary reservoirs and fluxes of P involve dissolved phosphate ion (PO43), dissolved organic P (DOP), and particulate inorganic P. The bulk of the P exists in the soil, marine sediments, and, of course, crustal rocks as apatite. Where the concentration of apatite is great enough, it can be commercially mined. The soil fraction is distributed approximately in proportion to the area of a region, ranging from 5900 Tg P in Europe to 29,400 Tg P in Tropical Africa, whereas the mineable rock is concentrated in North America (4,700 Tg P) and Tropical Africa (5,900 Tg P). The particulate soil fractions can be mobilized by erosional processes into rivers and subsequently to the oceans, and is estimated to be about 17 Tg P/yr. A lesser amount, about 4 Tg P/yr, is carried by the wind into the atmosphere; however, the residence time there is very short, and the atmospheric reservoir is only about 0.025 Tg P. The P cycle does not have an important atmospheric gaseous component, unlike C, N, or S.

Table 2.5 The major reservoirs and fluxes of the global phosphorus cycle. Three previous evaluations are compared, up-dated with new information, where possible, and a current estimate derived (summarized in Figure 2.4). The means of calculations and sources of each estimate are provided below. It must be remembered that all such calculations yield a value with considerable uncertainty, not a precise number. It is important to examine the underlying assumptions and sources of error for each term as given in the original reference

et al.
Up-date Reference

Particulates over land 0.025 (1)
Particulates over oceans 0.003 (1)
Biota 1950 3,000 1,805 2,600 (2)
Soil 200,000 160,000 96,000160,000 (3)
Mineable rock 31,000 9,920 3,1409,000 19,000 (4)
Fresh-water (dissolved) 90 90 (5)
Biota 124 138 128 50120 (6)
Dissolved (inorganic) 124,000 92,600 120128,000 80,000 (7)
Detritus (particulates) 650 (8)
Sediments 8.4 x 108 4 x 108 840,000,000 (9)

FLUXES (Tg P/yr)

Atmosphere (land) Atmosphere (ocean) 2 1.0 (1)
Atmosphere (land) Atmosphere (ocean) 0.3
Atmosphere Land 3.79.3 3.2 (1)
Atmosphere Ocean 2.63.5 1.4 (1)
Land Atmosphere ? 4.3 (1)
Ocean Atmosphere ? 0.3 (1)
Marine dissolved Biota 961 9921042 9901,300 6001,000 (10)
Marine detritus Sediment 1.9 1.7 13 213 (11)
Terrestrial biota Soils 229 63.6 136237 200 (12)
Mineable rock Soil 12.4 12.4 12.6 14 (14)
Soil Fresh-water 2.512.3 47 (13)
Fresh-water (diss.) Oceans 1.9 1.7 1.54 (14)
Fresh-water (part.) Oceans 17.4 17 (14)

(1) The atmospheric reservoir and fluxes are directly from Graham and Duce (1979), who summarized extensive measurements of atmospheric P concentrations and deposition rates in marine and continental regions.
(2) Estimates of the terrestrial biota are generally calculated from estimates of C mass of 4.5 x 105 Tg C (Bolin, 1979) 8.3 x 105 Tg C (Whittaker and Likens, 1975), and C/P atomic ratios of 500 (Stumm, 1973) to 833 (Deevey, 1970), though Pierrou (1976) used dry-weight biomass conversions. These yield a most-likely value of 2,600 Tg P, with a range of 1,4004,300 Tg P.
(3) Inorganic P in the soil has been computed from a total land area of 130 x 1012 m2 and a soil depth of 0.6 m (Lerman et al. 1975) to 1.0 m (Pierrou, 1976), with a P content of 0.100.12 % (Taylor, 1964) and a soil density of 1 kg/dm3.
In their calculation of soil volume, Lerman et al. apparently divided by 0.6 m rather than multiplying, yielding an over-estimate. Total soil P, including about 10% organic P (Bohn, 1976), is thus 96,000160,000 Tg P, depending on soil depth.
(4) The amount of P in mineable rock, as 30% of P2O5, has been defined as that minimum amount which is economically recoverable. As demand and technology increase, the P content of mineable rock decreases and the reservoir `increases'. The estimates of reservoir size and consumption rate are from Harris and Hare (1979) and FAO (1980).
(5) Pierrou (1976) calculated the P in fresh-waters from a total volume of 7.2 x 1014 m3 and a mean concentration of 0.12 g P/m3. The concentration value is probably uncertain by a factor of 2.
(6) Phosphorus in the marine biota is calculated generally from applying the Redfield ratio of C/P = 106 and carbon estimates of 2,000 Tg C (Williams, 1975) to 5,000 Tg C (Bolin, 1979). The upper estimate of Lerman et al. (1975) is based on out-dated production data.
(7) The previous estimates of dissolved inorganic P in the oceans have used mean concentrations of 0.080.10 g P/m3 and mean depths of 3,0003,500 m. More recent GEOSECS data (Takahashi et al. 1981) suggest a concentration of 0.062 g P/m3.
(8) The inventory of marine detrital P can be calculated as the amount of particulate carbon (3 x 104 Tg C; Mopper and Degens, 1978) times a detrital C/P atomic ratio of 120 (Broecker, 1974), or 650 Tg P.
(9) Stumm (1973) calculated the phosphorus in sediments from a geochemical mass balance, which is based on more recent data than the value presented in Lerman et al. (1975).
(10)  Estimates of the photosynthetic uptake by marine biota have been obtained by applying the Redfield ratio to productivity data, which range from 2.5 x 104 Tg C/yr to 4 x 104 Tg C/yr (De Vooys, 1979), or 6001000 Tg P/yr. The release of P by decomposition is assumed to be equal to photosynthetic uptake.
(11)  Emery et al. (1955) assumed that the P content of sediments is 0.092% and that 1 cm of solid sediment forms every 6000 years, for a rate of 13 Tg P/yr. Assumptions of steady state, as calculated by the others, indicate that this figure might be high, and that a value of 2 Tg/yr is more appropriate.
(12) The uptake and release of P by terrestrial biota has been estimated from productivity estimates of 3 x 104 C/yr (Bolin, 1979) to 5 x 104 Tg C/yr (Whittaker and Likens, 1975) and the C/P atomic ratios of 500822. A mean estimate of 200 Tg P/yr results.
(13) Phosphate is introduced into fresh-waters from natural leaching processes and from various human activities it is difficult to differentiate between the sources.
Pierrou used leaching rates from respective land-use types to estimate the total input. A similar analysis using the land-use types specified for the different geographic zones and up-dated loss rates for the different land uses (forest, grass, desert-swamp, and agriculture-pasture) from Rast and Lee (1978) gives the result 4 Tg P/yr.
(14) The river run-off of P to the oceans includes both natural and human-influenced leaching and particulate erosion products, less that which is retained or consumed within the river. The most recent calculation of dissolved export is described in Wollast (Chapter 14, this volume). Particulate export, assuming a ratio of 0.075% for the P content of total suspended sediments (e.g. Holland, 1978), is considerably greater.

Figure 2.4 A global phosphorus cycle. Fluxes are in Tg P yr-1 and reservoirs are in Tg P. From Table 2.5

The uptake and release of phosphate by terrestrial plants is about 200 Tg P/yr, while the equivalent marine flux is 6001000 Tg P/yr. Given the lesser marine biomass, the turnover rate in the oceans is much greater than on land. The amount finally sedimenting in the oceans is only a small fraction of the productionmineralization flux.

The natural cycle of P partly regulates the distribution of biomass because the supply and levels of phosphate are low relative to the requirements for plant and animal nutrition. Superimposed on the natural cycle is man's influence: the mining and consuming of phosphates by society, and the release of P in domestic and industrial effluents.

Table 2.6 The distribution of the terrestrial and fluvial components of the global P cycle (Table 2.5) according to major geopolitical zones. Means of calculation are described in the footnotes

Europe U.S.S.R Pac.
China L.
Africa ME

Mineable rock
Biota         Soil
Rock         Soil
Soil           Freshwater
0.6-1.5 0.04-0. 0.04-0.1
0.05-0.2 0.1-0.4 0.02-0.1

(1) The distribution of terrestrial biota and the uptake and release of P was calculated by determining the percent vegetation type by region 
      and weighting that by kg C/m2 from Whittaker and Likens (1975).
(2) The soil P distribution by region was apportioned by the percent land surface in each region.
(3) Mineable rock and fertilizer consumption were calculated by grouping the country-specific and regional data of Harris and Hare (1979) and 
     FAO (1980) to the appropriate zones.
(4) The relative land cover in each zone was calculated, and the run-off ratios from Rast and Lee (1978) applied.
(5) Calculated by river zone by establishing the range of P concentrations for the rivers of that zone time their total water discharge.

The annual rate of fertilizer consumption in the industrialized regions is now about 10% of the steady-state flux between the soil and biota, and approaches 50% in Europe. In the less-developed regions the rate is lower. The total input of P to fresh-waters which also includes wastes, is a considerable fraction of the fertilizer consumption. This suggests, that the mobility of this `excess' P is sufficient to be transported away from its sites of application. Most of this phosphorus is discharged into coastal waters.

Although adequate data on the pre-industrialization loads of P in rivers do not exist, there has been at least a several-fold increase (Wollast, Chapter 14, this volume). The eutrophication of lakes and coastal waters is well-known. This phenomenon also alters the cycles the C and N. Given the above observations, the P cycle is thus significantly perturbed by the activities of man. Figure 2.4 gives an overview of the reservoirs and fluxes of the global P cycle.



A number of attempts have been made over the last 20 years to assess the Earth's reservoirs of sulphur and the natural and man-made transfers to determine how seriously the balance of nature was being affected (Eriksson, 1960; Junge, 1963; Robinson and Robbins, 1970; Kellogg et al., 1972; Friend, 1973; Cadle, 1975; Granat et al., 1976; Cullis and Hirschler, 1980; Ivanov and Freney, 1983). These reports show that man-made emissions are large and seriously interfere with the natural sulphur cycle. As the sulphur cycle clearly interacts with those of carbon, nitrogen and phosphorus, the main features of these reports are summarized and discussed to serve as background information for the study on cycle interactions.

The most recent of these reports (Ivanov and Freney, 1983) includes the results from work done in the Soviet Union that has not previously been readily available. This report provides up-to-date information on many fluxes and attempts to separate, wherever feasible, the natural and man-made contributions to these fluxes. The following summary is largely based on it. In order to make the presentation short, no indications of the uncertainties appropriate to the different values are included; this does not imply that the values are accurately known. For a further discussion of this aspect the reader is referred to the original work.

Table 2.7 summarizes the current information on the distribution of sulphur between the various spheres. The bulk of sulphur is contained in the lithosphere (24 x 109 Tg S) and the hydrosphere (1.3 x 109 Tg S), moderate amounts occur in the pedosphere and only small amounts (4.6 Tg S) are found in the atmosphere at any one time.

Table 2.7 Major sulphur reservoirs (Tg s)

Atmosphere(1) Tg S

Sulphate in aerosols 0.7
Sulphur dioxide 0.5
Carbonyl sulphide 2.3
Other reduced sulphur gases 0.8
Total in troposphere 4.3
Stratosphere 0.5


Hydrosphere (2)

Ocean water 1.3 x 109

1.3 x 109

Lithosphere (3)

Continental and subcontinental
Sedimentary 5.2 x 109
Granite 7.8 x 109
Basalt 8.8 x 109
Sedimentary (layer I) 0.3 x 109
Tholeitic and olivine basalt (layer II) 0.6 x 109
Layer III 1.6 x 109

24.3 x 109


Soil(4) 2.6 x 105
Soil organic matter (5) 1.1 x 104
Land plants(4) 760

2.7 x 105

(1) Ryaboshapko (1983)
(2) Volkov and Rozanov (1983)
(3) The lithosphere refers to the crust of the Earth. Migdisov et al. (1983)
(4) Krauskopf (1967)
(5) Freney and Williams (1983)

Within the lithosphere most of the sulphur occurs in rocks of the present continents and sub-continents (22 x 109 Tg S) and the major forms are metal sulphides and sulphates. The maximum concentration of reduced sulphur is found in argillaceous rocks of platform areas (0.4% S) and the minimum concentration occurs in effusive rocks (0.04% S). Sulphate sulphur varies from fractions of one per cent in humic pelagic formations to solid sulphate layers in evaporites (Migdisov et al., 1983).

Figure 2.5 The major natural and anthropogenic (circled) fluxes of the global biogeochemical sulphur cycle (Tg S/year) (from Ivanov and Freney 1983)

Roman figures denote:

 I output of sulphur-containing minerals;
II industrial treatment of the sulphur-containing raw materials; 
III inland water bodies;
IV volcanoes. 


P1 fuel combustion and metal smelting; P2 and P17 volcanic emission; P3 aeolian dust; P4 biogenic emission from land; P5 sea air-land air transport; P6deposition of large dust particles; P7 wash-out and dry deposition; P8 land air-sea air transport; P9 weathering; P10 river runoff to oceans; P1l transport to ocean in underground water; P12 runoff to inland water bodies; P13 input in fertilizers; P14 - leaching of fertilizers; P15 efflux from chemical industries; P16 efflux of acid mine water; P18 abrasion of shores; P19 - sea spray; P20 wash-out and dry deposition; P21 sedimentation of reduced sulphur; P22 sedimentation of sulphate; P23 biogenic emission from oceans.

Table 2.8 Sulphur transfer between the atmosphere and the earth's surface according to different authors(1) (Tg S/yr)

Flux Eriksson
and Robbins
Kellogg et al.
Granat et
Ivanov and

Combustion, smelting, etc. 39 40 70 50 65 65 113
Volcanic gases 1.5 2 3 28
Aeolian emission 0.2(2) 20
Biogenic gases from land 77 70 68

58 5 16
Biogenic gases from coastal regions 190

160 30 90

48 27 20
and the open ocean
Sea spray 44 44 43 44 44 140
Emission of long-lived reduced  5
     sulphur compounds
Uptake of SO2 by land surfaces
      and vegetation 77 70 26 15 15 28 17
Wash-out over land 57 55 70 86 86 43 51
Dry deposition of sulphate over land 15 20 10 20 16
Uptake of SO2 by ocean 70 70 25

25 10 11
Wash-out over ocean 146 60(3)

71 72

71 63 230
Dry deposition of sulphate over ocean 17

(1) From Ryaboshapko (1983)
(3) Excluding sea salt

In the pedosphere the bulk of the sulphur occurs in organic compounds in soil and in living plants (1.2 x 104 Tg S) while in the ocean, inorganic sulphate predominates. Carbonyl sulphide appears to be the dominant form of sulphur in the atmosphere.

From Figure 2.5, the biggest transfers of sulphur occur between: the atmosphere and the ocean due to wash-out and dry deposition (258 Tg S/yr); the pedosphere and the ocean in run-off (208 Tg S/yr); the ocean and the atmosphere in sea spray (140 Tg S/yr); the lithosphere and the hydrosphere by weathering (114 Tg S/yr); and the lithosphere and the atmosphere by fuel combustion and metal processing (113 Tg S/yr). It should be pointed out that accumulation of sulphur in the pedosphere has been neglected.

Table 2.8 shows a comparison between different estimates of the global sulphur fluxes affecting the atmosphere. To a certain degree the differences are due to the better data available during recent years and also reflect the large uncertainties still associated with many of the fluxes, and the changes with time in some of the fluxes. For example, there has been a very substantial rise in the anthropogenic emission of sulphur to the atmosphere between 1950 and the mid-seventies (Figure 2.6). During the latest years the increase has probably halted. It should be noted that the emission estimates, on which Figure 2.6 is based, are associated with considerable uncertainties. Recent estimates of sulphur emissions in Europe, U.S.S.R. and North America (ECE, 1981) give a figure of about 50 Tg S/yr. If this figure were correct, it seems unlikely that the global total at present would exceed 100 Tg S/yr.

At the present time the anthropogenic input most probably outweighs the natural input to the atmosphere over land and is approximately equal to the natural transfer of sulphur from the pedosphere to the oceans in river run-off. The total amount of sulphur in continental water bodies has nearly doubled due to man's activity.

An even more dramatic anthropogenic impact on the sulphur cycle is evident if the industrialized regions are considered separately. Within those regions (the eastern part of North America, Europe and parts of eastern Asia) emissions into the atmosphere have probably increased by at least one order of magnitude due to man's interference (Galloway and Whelpdale, 1980).

Due to the limited atmospheric residence time of the emitted sulphur, much of it is deposited within the same regions. As a result of these emissions, the chemical composition of air and of precipitation in industrial regions has been greatly affected (Rodhe, 1981). The deposition of sulphuric (and nitric) acid is now having a significant effect on water quality and maybe also on soil chemistry in those areas (Cook, Chapter 12, this volume).

Figure 2.6 Estimates of the global emission of sulphur from anthropogenic sources between 1860 and 1980 (Ryaboshapko, 1983)


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